After the withdrawal of the Machinists union’s petition for an election to organize flight attendants at Delta Air Lines, the National Mediation Board is referring the matter to the U.S. Justice Department for review, the airline said.

A union submits cards signed by at least 50 percent of workers eligible to vote on unionization in order to apply for an election to organize workers.

The National Mediation Board, which governs labor relations at airlines, wrote in a letter to Delta and the International Association of Machinists union Thursday that it “has reason to believe that some unknown person or persons knowingly submitted authorization cards with fraudulent signatures in possible violation of federal law,” according to the airline. The International Association of Machinists had said Monday that it withdrew its application for an election because it recently found that some of the cards submitted to authorize an election had insufficient information or questionable signatures.

Delta’s chief human resources officer Joanne Smith said in a written statement: “At the time of the IAM filing in January, many flight attendants were raising questions regarding the validity of the authorization cards the IAM submitted to the NMB.”